The UK’s appetite for 125% proof testing tower cranes does not come from some urge to obey the law, according to Tim Watson, tower crane consultant for the Construction Plant-hire Association (CPA). He spoke at an open meeting of the CPA’s Tower Crane Interest Group in January that attracted about 50 people.
A proof load stresses a tower crane like a big fish on a fishing rod. Opinions differ as to whether this test is valuable or unneccessary.
The LOLER regulations that govern crane use in the UK mention only a thorough examination after installation and before first use, and after assembly and before first use. The LOLER approved code of practice leaves the choice of doing a test—any test—up to a competent person, he says.
Only British Standard BS 7121-2:2003 requires overload testing for cranes that are not thoroughly examined by a competent person after erection or alteration, but, according to Watson, this is not law. Its proof-load test is a two-step process. First, the operator raises the rated load, and continues to raise it until every part of the mechanism has been exposed to the load. Then this test is repeated with 125% of rated capacity. It concludes that cranes which cannot manage 125% loading are unsuitable for the job, Watson says.
A completely different picture emerges from the 2006 European standard for the manufacture of tower cranes, EN 14439, published in the UK in January 2007. It defines what manufacturers should do before their equipment is used for the first time. These include a dynamic test of 110% rated capacity, and, for cranes from 1t to 100t capacity, a one-time-only static proof load test of 125%, he says.
According to Watson, it is unclear whether manufacturers would perform an overload test on a tower crane before delivery, because they are modular manufacturers, who merely supply a bunch of components rather than a complete crane.
The other potential problem with the standard as written, Watson says, is that rated capacity limiters on many cranes cut out crane motions at 110% of capacity. In other words, a dynamic test does not really test the crane, since it could, in extreme circumstances, work at that level normally.
For some tower crane operators, an overload test is just confirmation of lifting capacity. “We set the cranes up per the manufacturers’ recommendations,” says Dennis Bates, vice president, tower division, AmQuip Corp, Pennsylvania, USA. “Most call for the test to be 10% over the rated capacity which we do here. The main reason for this is to insure that the crane will pick the rated capacity. This 10% compensates for the static and dynamic loading of the machine as you pick the load.”
A 125% proof overload test should not be carried out too often, argues Nick Fraser, Imes cranes and lifting equipment chief engineer. The Aberdeen, Scotland-based Imes group includes the Water Weights crane testing business, which uses huge water-filled bags as proof loads.
Diverging opinions
Others argue that anything less than a 125% proof load is inferior. “I’ll bet I know 15 inspectors in Dubai who are afraid to put a 125% proof load on a crane because they might break it, or it might tip over,” says Mike Watson, a retired American Bureau of Shipping crane inspector.
An audience member at the UK tower crane meeting said that cranes should be proof tested at 125%, because they need to be built to deal with out-of-service loading. Wind loads at night can exceed 125% of rated capacity easily.
According to Tim Watson, the 125% proof load test comes from standards dating back to the 1960s. Watson says that the 125% proof test gives comfort to contractors and crane operators. It tells them that even if the crane is slightly overloaded, it will remain standing. But, he asks, is this a false sense of comfort?
He says that that a thorough examination and functional test is more important than an overload test. The most common problems on cranes are due to fatigue. Sticking on 25% or 50% extra weight will not show up fatigue. And, he asks, what is the point of an overload test? If you do find the crane has problems, it’s a disaster for the site schedule.
An audience member who did not give his name replied that on two occasions he found cracks on tower cranes during a 125% proof test, and that he was glad to have that information before people started to work with it.
